1. Field of the Invention
The invention is a catalytic process for cracking paraffin fractions to light olefins and naphtha. More particularly the invention is a process for converting paraffins to gasoline by fluid catalytic cracking (FCC).
2. Description of Related Methods in the Field
In the fluid catalytic cracking (FCC) process a petroleum derived hydrocarbon charge stock is contacted with a fluidized finely divided catalyst in a reaction zone. The catalyst is fluidized by means of a lift gas. The charge stock is converted by cracking to lower boiling hydrocarbons and coke. The lower boiling hydrocarbon vapor and spent catalyst are separated in a containment vessel, termed in the art the reactor vessel. Separated spent catalyst is steam stripped of entrained vapor and the remaining spent catalyst coated with a layer of unstrippable coke is passed from the reactor vessel to a catalyst regenerator vessel. There, spent catalyst is regenerated by controlled oxidation of the coke coating to carbon dioxide and carbon monoxide. A hot, active regenerated catalyst is returned to the reaction zone.
Separated lower boiling hydrocarbon vapor, stripped vapor and spent stripping steam is withdrawn from the reactor vessel and passed to a fractionation train where cracked hydrocarbon vapors are separated by fractional distillation into the desired intermediate fractions. Any number of intermediate fractions can be made based on refinery configuration and product demand. For example, product fractions may include a gaseous fraction, naphtha, kerosene, diesel oil, gas oil and vacuum gas oil. Of these fractions, the naphtha fraction is the most desirable because of its use as an automobile fuel blending component. The intermediate fractions comprising kerosene and diesel oil may be used for their fuel value. In the alternative they may be processed to produce additional gasoline blending components. The heavy fractions comprising gas oil and vacuum gas oil may be used for the production of heavy fuel oil. Optionally, a portion of the heavy fraction may be recycled to the fluid catalytic cracking reaction zone to produce additional lower boiling hydrocarbons, including an additional increment of naphtha for gasoline blending.
U.S. Pat. No. 4,422,925 to D. Williams et al. teaches a process for the fluid catalytic cracking (FCC) of a plurality of hydrocarbon feedstocks. In the process a gaseous paraffinic hydrocarbon is used as a lift gas to fluidize a cracking catalyst in a riser (transfer line) reactor. Naphtha and gas oil feedstocks are cracked to yield liquid fuels.
U.S. Pat. No. 3,894,932 to H. Owen teaches a fluid catalytic cracking process for converting a gas oil with Y faujasite catalysts. The catalyst is first suspended in a C.sub.3 -C.sub.4 gaseous hydrocarbon fraction and then contacted with a higher boiling hydrocarbon fraction.
U.S. Pat. Nos. 4,479,870; 4,541,922 and 4,541,923 disclose the use of lift gas in a fluid catalytic cracking (FCC) riser reactor. Certain desirable results are achieved by selecting the composition of the lift gas.